Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
  • G01V3/30—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with electromagnetic waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/12—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with electromagnetic waves
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
  • G01V3/26—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with magnetic or electric fields produced or modified either by the surrounding earth formation or by the detecting device
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/38—Processing data, e.g. for analysis, for interpretation, for correction

Definitions

• Geologic formations below the surface of the earth may contain reservoirs of oil and gas, which are retrieved by drilling one or more boreholes into the subsurface of the earth.
• the boreholes are also used to measure various properties of the boreholes and the surrounding subsurface formations.
• Exemplary techniques include using ferrite and copper shielding, using reference signal for calibration purposes and using asymptotic behavior of the conductive collar time response to filter out the collar signal.
• the conductive collar signal is typically more than two orders of magnitude greater than the formation signal even if ferrite and copper shields are used. Then the accuracy of bucking and filtering may not be sufficient to facilitate measurements.
• a method of processing electromagnetic signal data includes: disposing a downhole tool in a borehole in an earth formation, the downhole tool including a conductive carrier, a transmitter, a first receiver disposed at a first axial distance from the transmitter, and a second receiver disposed at a second axial distance from the transmitter that is less than the first axial distance; performing a downhole electromagnetic operation, the operation including transmitting an electromagnetic (EM) signal from the transmitter into the formation and detecting a first EM response signal by the first receiver and a second EM response signal by the second receiver; applying a linear transformation to the second EM response signal to generate a transformed signal, the linear transformation having parameters associated with a set of data corresponding to a signal representing the conductive carrier; and subtracting the transformed signal from the first EM response signal to generate a corrected EM signal.
• EM electromagnetic
• An apparatus for processing electromagnetic signal data includes: a downhole tool configured to be disposed in a borehole in an earth formation, the downhole tool including a conductive carrier, a transmitter, a first receiver disposed at a first axial distance from the transmitter, and a second receiver disposed at a second axial distance from the transmitter that is less than the first axial distance; and a processor configured to perform: receiving a first electromagnetic (EM) response signal from the first receiver and a second EM response signal from the second receiver in response to an EM signal transmitted into the formation from the transmitter; applying a linear transformation to the second EM response signal to generate a transformed signal, the linear transformation having parameters associated with a set of data corresponding to a signal representing the conductive carrier; and subtracting the transformed signal from the first EM response signal to generate a corrected EM signal.
• EM electromagnetic
• FIG. 1 depicts an exemplary embodiment of a drilling, formation evaluation and/or production system
• FIG. 2 depicts an exemplary embodiment of a downhole tool
• FIG. 3 depicts a structure representing an exemplary configuration of the downhole tool of FIG. 2 in an earth formation
• FIG. 4 depicts exemplary transient electromagnetic responses obtained in the presence of a typical conductive pipe
• FIG. 5 is a flow chart providing an exemplary method of processing electromagnetic signal data and/or measuring formation properties
• FIG. 6 depicts a model including time-domain signals from a first and second receiver placed on a conductive drill collar with no surrounding conductive media;
• FIG. 7 depicts residuals of a drill collar signal after signal elimination from a first receiver signal of FIG. 6 using an impulse response function;
• FIG. 8 depicts corrected electromagnetic signal data
• FIG. 9 depicts the signal data of FIG. 8 over a time interval of interest.
• Apparatuses and methods are provided for reducing and/or eliminating parasitic signal data due to downhole components (e.g., conductive drill collars, borehole strings or tool components) from electromagnetic (EM) measurement data.
• the apparatuses and methods described herein are utilized with transient EM operations, such as ultra-deep resistivity measurement while drilling.
• An exemplary method is based on acquiring EM signals from at least a first and second EM receiver that are axially spaced downhole relative to an EM transmitter.
• a first EM signal is generated from the first receiver and a second EM signal is generated from the second receiver located closer to the transmitter.
• the second receiver is combined with a coefficient or function to generate a transformed signal, which can be subtracted from the first EM signal to generate a corrected EM signal that is free (or at least substantially free) of the parasitic signal.
• a linear transformation of the second signal is performed with parameters that are adjusted based on acquiring a set of data representing at least substantially only the drill collar signal.
• the linear transformation is a convolution of the second EM signal with an impulse response function having two parameters.
• the transformed signal is subtracted from the first signal to generate an EM signal that has at least substantially all of the influence from the conductive component removed, without requiring data extrapolation.
• an exemplary embodiment of a well drilling, logging and/or production system 10 includes a borehole string 12 that is shown disposed in a wellbore or borehole 14 that penetrates at least one earth formation 16 during a drilling or other downhole operation.
• borehole or “wellbore” refers to a single hole that makes up all or part of a drilled well.
• formations refer to the various features and materials that may be encountered in a subsurface environment and surround the borehole.
• a surface structure 18 includes various components such as a wellhead, derrick and/or rotary table or supporting the borehole string, lowering string sections or other downhole components.
• the borehole string 12 is a drillstring including one or more drill pipe sections that extend downward into the borehole 14, and is connected to a drilling assembly 20.
• system 10 includes any number of downhole tools 24 for various processes including formation drilling, geosteering, and formation evaluation (FE) for measuring versus depth and/or time one or more physical quantities in or around a borehole.
• the tool 24 may be included in or embodied as a bottomhole assembly (BHA) 22, drillstring component or other suitable carrier.
• BHA bottomhole assembly
• a “carrier” as described herein means any device, device component, combination of devices, media and/or member that may be used to convey, house, support or otherwise facilitate the use of another device, device component, combination of devices, media and/or member.
• Exemplary non-limiting carriers include drill strings of the coiled tubing type, of the jointed pipe type and any combination or portion thereof.
• Other carrier examples include casing pipes, wirelines, wireline sondes, slickline sondes, drop shots, downhole subs, bottom-hole assemblies, and drill strings.
• the tool 24, the BHA 22 or other portions of the borehole string 12 includes sensor devices configured to measure various parameters of the formation and/or borehole.
• the sensor devices include one or more transmitters and receivers configured to transmit and receive electromagnetic signals for measurement of formation properties such as composition, resistivity and permeability.
• An exemplary measurement technique is a transient EM technique.
• the tool 24, BHA 22 and/or sensor devices include and/or are configured to communicate with a processor to receive, measure and/or estimate directional and other characteristics of the downhole components, borehole and/or the formation.
• the tool 24 is equipped with transmission equipment to
• Such transmission equipment may take any desired form, and different transmission media and connections may be used. Examples of connections include wired, fiber optic, acoustic, wireless connections and mud pulse telemetry.
• the processor may be configured to receive data from the tool 24 and/or process the data to generate formation parameter information.
• the surface processing unit 28 is configured as a surface drilling control unit which controls various drilling parameters such as rotary speed, weight-on-bit, drilling fluid flow parameters and others.
• the tool 24 is configured as a downhole logging tool. As described herein, "logging" refers to the taking of formation property measurements.
• Examples of logging processes include measurement-while-drilling (MWD) and logging- while-drilling (LWD) processes, during which measurements of properties of the formations and/or the borehole are taken downhole during or shortly after drilling. The data retrieved during these processes may be transmitted to the surface, and may also be stored with the downhole tool for later retrieval. Other examples include logging measurements after drilling, wireline logging, and drop shot logging.
• MWD measurement-while-drilling
• LWD logging- while-drilling
• Other examples include logging measurements after drilling, wireline logging, and drop shot logging.
• FIG. 2 illustrates an embodiment of the downhole tool 24.
• the downhole tool 24 is disposed in a carrier such as a housing 30.
• the housing is incorporated as or in a downhole component such as a borehole string section, a drill pipe or a drill collar.
• the housing 30 and/or other component are typically made from a conducting material such as steel.
• the tool 24 includes a resistivity measurement assembly 32 incorporating at least one electromagnetic (EM) source and multiple EM receivers.
• An EM transmitter 34 e.g., a transmitter antenna or coil
• An electric source 40 which may be disposed downhole or at a surface location, is configured to apply electric current to the transmitter 34.
• the measurement assembly 32 is configured to perform an inductive transient EM measurement operation.
• the source 40 applies transient pulses of current to the transmitter 34, which induces current in the formation 16.
• the current generates a magnetic field that is detected by the receivers 36 and 38.
• the tool 24 utilizes electromagnetic measurements to determine the electrical conductivity of formations surrounding the borehole.
• Various types of tools may be employed to measure formations at various "depths of investigations" or DOI, which correspond to distances from the tool and/or borehole in a direction perpendicular to an axis of the tool and/or borehole (e.g., the Z axis of FIG. 2), referred to herein as "radial distances.”
• Transient EM methods are particularly useful for ultra-deep investigations (e.g., radial distances of 10s to hundreds of meters from the tool and/or borehole).
• voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation.
• the transient electromagnetic field is sensitive only to remote formation zones and does not depend on the resistivity distribution in the vicinity of the transmitter.
• the transmitter and the receivers are disposed axially relative to one another.
• An "axial" location refers to a location along the Z axis that extends along a length of the tool 24 and/or borehole 14.
• the first receiver 36 is positioned at a selected axial distance LI from the transmitter 34, and the second receiver 38 is positioned at a shorter axial distance L2 from the transmitter.
• the first and second distances are selected to have a specific ratio, e.g., LI is twice that of L2.
• the receivers 36 and 38 are identical or at least substantially identical, such that they would measure the same signal if the receivers are disposed at the same axial and radial location.
• the receivers 36 and 38 each have the same (or at least substantially the same) configuration parameters.
• Such parameters include the number and diameter of coil windings, the coil material, the effective area, the magnetic field to voltage conversion factor and/or voltage gain.
• FIG. 3 shows an exemplary structure representing a configuration of the tool 24 with the formation 16.
• the structure includes a first zone 42 substantially defined by a metal drill collar, pipe or other conductive carrier with conductivity o'j , a transition layer 44 having a conductivity ⁇ 3 ⁇ 4 and a remote formation layer 46 having a conductivity 03.
• the magnetic permeability of the entire space is ⁇ .
• the boundary 48 separating the metal carrier from the transition layer and the boundary 50 separating the regions of transition layer and remote formation share a common Z-axis.
• the radius of boundary 48 is labeled as r lti d
• the radius of boundary 50 is labeled as r t i.
• An electromagnetic field is excited by the transmitter current loop 34 of radius, r xt , and is measured by receivers 36 and 38 of radius r X .
• FIG. 4 shows exemplary transient responses obtained in the presence of a typical conductive pipe.
• Curves 51, 52 and 53 indicate responses at radial distances (perpendicular to the Z axis) of 1, 2, and 4 meters respectively to a remote boundary (e.g., boundary 204).
• Response curve 54 represents the response to a remote boundary at an infinite distance.
• Response curve 54 is nearly indistinguishable from and overlaps response curves at a distance of 6, 8 and 10 meters.
• FIG. 4 illustrates the fact that at late times corresponding to deep investigation, the conductive pipe signal typically dominates the transient response of the earth's formations by at least an order of magnitude.
• FIG. 5 illustrates a method 60 for measuring parameters of an earth formation using electromagnetic signal measurements.
• the method also includes processing and/or analyzing received signals to reduce and/or eliminate the signal corresponding to conductive downhole components such as drill collars or drill pipes from EM data, such as transient EM data.
• the method 60 includes one or more of stages 61-65 described herein. The method may be performed continuously or intermittently as desired. The method is described herein in conjunction with the tool 24, although the method may be performed in conjunction with any number and configuration of processors, sensors and tools. The method may be performed by one or more processors or other devices capable of receiving and processing measurement data.
• the method includes the execution of all of stages 61-65 in the order described. However, certain stages 61-65 may be omitted, stages may be added, or the order of the stages changed.
• the tool 24 is lowered in the borehole.
• the tool 24 may be lowered, for example, during a drilling operation, LWD operation or via a wireline.
• each receiver signal can encompass one or multiple signals over one or more time intervals.
• receiver signal data is acquired over a time that exceeds a time period of interest for formation signals. This includes a time interval from t; (the first time of interest for the formation signal) to ti ftbe end of a time interval over which the drill pipe signal is dominant).
• a first time interval ⁇ ti,t 2 ⁇ includes a signal that is responsive to both the formation property and to a drill pipe or other conductive component, and a second time interval iti, ⁇ includes a signal that is dominated by or responsive almost entirely to the drill pipe.
• the second time interval can be determine by, e.g., calibration data taken at the surface, prior measurement data or known intervals,
• a transformation is applied to the second receiver signal R2 to generate a transformed signal.
• the transformed signal is then subtracted from the first receiver signal Rl to generate a corrected signal that is entirely or at least substantially entirely free of the portion of the first signal due to the conductive drill pipe or other downhole component.
• the second receiver signal R2 is transformed by multiplying the receiver signal R2 by some coefficient.
• the coefficient may be a constant based on, e.g., a ratio between the distance from Rl to the transmitter (T) and the distance from R2 to the transmitter T.
• An exemplary ratio is (R1-T) 3 /(R1-R2) 3 , where Rl-T is the distance from Rl to T and R1-R2 is the distance from Rl to R2.
• the transformation is a linear transformation having parameters that are adjusted based on acquiring a set of data at a late time interval ⁇ t3, i ⁇ during which the received signal contains at least substantially no portion, or at least less than a tolerable systematic error, of the formation signal, therefore representing the drill collar signal only.
• This late time interval can be ascertained by experimentation, previous measurement data or other knowledge indicating at what time the collar signal dominates the receiver signal.
• the linear transformation is a convolution with an impulse response function having two or more parameters.
• An exemplary impulse response function is a function having parameters that include the second signal (R2) and a time value (or multiple time values corresponding to sampling points) during a selected time interval.
• the parameters are taken from the second receiver data taken at the late time interval ⁇ tj,t4 ⁇ .
• FIG. 6 shows a model including time-domain signals Rl and R2 from two receivers 36 and 38 placed on a conductive drill collar with no conductive media surrounding the tool 24.
• the modeling represented in FIG 6 was created for two receivers having a Z-axis direction of sensitivity. The receivers are spaced at different locations on the Z axis relative to the transmitter; Rl corresponds to receiver 36 spaced 10m from the transmitter 34, and R2 corresponds to receiver 38 spaced 5m from the transmitter.
• the transmitter magnetic dipole moment was 50Am 2 and the effective area of the receivers were 10 m 2 .
• the model results represent an initial calibration condition where the two receivers are used to zero out the signal from the drill collar.
• the model is provided to illustrate a derivation of the impulse response function and demonstrate how the convolution isolates the signal from the drill collar or other conductive component.
• An impulse response in the time domain is defined as:
• R 1 (t) h(t - T) - R 2 (T)dT , (1)
• first receiver response R l (t) in this model is defined as a convolution of the impulse response h(t) and the second receiver response R 2 (t) .
• ⁇ corresponds to some time delay.
• the corresponding equation in the frequency domain (s-domain) is:
• R 1 (s) H(s) - R 2 (s) , (2) where H(s) is the Laplace transform of the impulse response h(t) .
• R 2 (s) K 2 (s) - T(s) , (4)
• T(s) is the s-domain transmitter signal (magnetic field)
• K l (s) and K 2 (s) are the transmitter-receiver transfer functions.
• R 2 (s) and correspondingly h(t) does not depend on the transmitter spectrum and is therefore immune to transmitter noise and instability.
• the impulse response is approximated by a function h * (t) with a limited number of parameters that are relatively easy to control.
• the parameters vector is determined from the following system of equations:
• FIG 7 shows residuals of the drill collar signal after signal elimination using impulse response (6), demonstrating that convolution of the impulse response and the second signal R2 yields a transformed signal that represents substantially all of the drill collar portion of the first receiver signal Rl .
• the relative error shown in FIG. 7 is defined as:
• the linear combination of the receiver signals i.e., subtraction of the transformed signal, provides a corrected signal S(t) that is at least substantially free of the drill collar signal.
• the corrected signal can be represented by:
• the corrected signal is not only at least substantially free from the drill collar signal, but also compensates for various downhole conditions.
• Exemplary conditions include temperature effects on of the transmitter dipole and waveform and the effective area of the receiver coils, transmitter instability, receiver electronics temperature drift and other instabilities associated with the downhole environment.
• a calibration while drilling is performed. This can be done by utilizing measurements in an auxiliary late time acquisition interval where the formation signal can be neglected compared to the conductive drill collar signal, e.g., the late time interval . Data in this interval is used to adjust parameters of the linear transformation, e.g., the parameters x l and 2 ofthe impulse response function h *(t).
• properties of the formation are estimated based on the processed and corrected receiver signal Rl .
• FIGS. 8-9 illustrate an example of the benefit of using transformation parameters based on data acquired downhole relative to data acquired during an uphole calibration.
• FIGS. 8-9 show corrected data as affected by bucking instability that causes 2% of a residual drill collar signal in a corrected signal S(t) .
• FIGS. 8-9 include synthetic data obtained for the tool configuration shown in FIG. 3.
• the formation signal used for the model corresponds to a homogeneous resistivity of 10 Ohm ⁇ m .
• Data line 74 corresponds to the signal S(t) calculated according to equation (10) using parameters of the impulse response determined during uphole calibration.
• the ideal data line 78 represents the formation signal calculated for equivalent transmitter and receiver dipoles for a homogeneous conductive medium with resistivity 10 Ohm ⁇ m . This signal can be considered the true formation signal. It can be seen from the FIGS. 8-9 that without the calibration using downhole operation data, the residual drill collar signal due to its instability dominates the measured signal in the time interval 10 4 - 10 ⁇ 3 s.
• the time interval of 10 4 - 10 ⁇ 3 s typically corresponds to a depth of investigation of about 60-100 feet, which is the most important range for ultra-deep geo-steering (including ahead of the bit) applications.
• the apparatuses and methods described herein provide various advantages over prior art techniques.
• the apparatuses and methods allow for removing the effects of the drill collar without having to know the changes in the drill collar that occur during downhole operation.
• Such changes include environmental changes (temperature and pressure) as well as physical changes such as deformation and vibration.
• the systems described herein may be incorporated in a computer coupled to the tool 24.
• Exemplary components include, without limitation, at least one processor, storage, memory, input devices, output devices and the like. As these components are known to those skilled in the art, these are not depicted in any detail herein.
• the computer may be disposed in at least one of a surface processing unit and a downhole component.
• various analyses and/or analytical components may be used, including digital and/or analog systems.
• the system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, pulsed mud, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art.
• teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention.
• ROMs, RAMs random access memory
• CD-ROMs compact disc-read only memory
• magnetic (disks, hard drives) any other type that when executed causes a computer to implement the method of the present invention.
• These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure.

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